14 CFR 63.35(d):
d) An applicant for a flight engineer certificate or rating must have passed the written tests required by paragraphs (a) and (b) of this section since the beginning of the 24th calendar month before the month in which the flight is taken. However, this limitation does not apply to an applicant for a flight engineer certificate or rating if -
(1) The applicant -
(i) Within the period ending 24 calendar months after the month in which the applicant passed the written test, is employed as a flight crewmember or mechanic by a U.S. air carrier or commercial operator operating either under Part 121 or as a commuter air carrier under Part 135 (as defined in Part 298 of this title) and is employed by such a certificate holder at the time of the flight test;
(ii) If employed as a flight crewmember, has completed initial training, and, if appropriate, transition or upgrade training; and
(iii) Meets the recurrent training requirements of the applicable part or, for mechanics, meets the recency of experience requirements of Part 65; or
(2) Within the period ending 24 calendar months after the month in which the applicant passed the written test, the applicant participated in a flight engineer or maintenance training program of a U.S. scheduled military air transportation service and is currently participating in that program.
(e) An air carrier or commercial operator with an approved training program under Part 121 of this chapter may, when authorized by the Administrator, provide as part of that program a written test that it may administer to satisfy the test required for an additional rating under paragraph (b) of this section.
